November 2011 - kernelnewbies@kernelnewbies.org

Spinlocks and interrupts
by Kai Meyer Nov. 14, 2011

Nov. 14, 2011

When I readup on spinlocks, it seems like I need to choose between disabling interrupts and not. If a spinlock_t is never used during an interrupt, am I safe to leave interrupts enabled while I hold the lock? (Same question for read/write locks if it is different.) -Kai Meyer

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put a message and wait a key on kernel boot
by Albert Wesker Nov. 14, 2011

Nov. 14, 2011

i am trying to put a message editing the file ¨e820.c¨ to put i use printk(), but i dont know how do for wait until the user press a key

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FIQ support in Linux Kernel
by Konstantin Zertsekel Nov. 14, 2011

Nov. 14, 2011

Intro: basically, I tried to post this question to linux-arm-kernel-request(a)lists.arm.linux.org.uk, but the email just fails to reach the server or something... :-( Following the FIQ disscussion here: http://www.spinics.net/lists/arm-kernel/msg14959.html and here: >> /*============================================================================= >> * Undefined FIQs >> *----------------------------------------------------------------------------- >> * Enter in FIQ mode, spsr = ANY CPSR, lr = ANY PC >> * MUST PRESERVE SVC SPSR, but need to switch to SVC mode to show our msg. >> * Basically to switch modes, we *HAVE* to clobber one register... brain >> * damage alert! I don't think that we can execute any code in here in any >> * other mode than FIQ... Ok you can switch to another mode, but you can't >> * get out of that mode without clobbering one register. >> * >> * Fast Interrupt dispatcher >> * Enter in FIQ mode, spsr = SVC/USR/IRQ/dABT CPSR, lr = SVC/USR/IRQ/dABT PC >> */ > Subject: Linux crashes with FIQs enabled > From: linux(a)arm.linux.org.uk (Russell King - ARM Linux) > Date: Fri, 15 Aug 2003 18:00:33 +0100 > In-reply-to: <007201c3634c%242b61cd40%243f00a8c0(a)geomation.com>; from registration(a)junck.com on Fri, Aug 15, 2003 at 10:42:03AM -0600 > References: <007201c3634c%242b61cd40%243f00a8c0(a)geomation.com> > It looks like you failed to read my comments above - this may be why > you're seeing the occasional problem. > Think what happens when you've just entered SVC mode (eg, due to a SWI) and > you haven't saved any state. The previous context is saved in SPSR_svc, > and the processor has disabled IRQs for you, but not FIQs. > You take a FIQ from SVC mode before you've saved any state. The SVC-mode > CPSR gets saved in SPSR_fiq. What about SPSR_svc? How are you saving/ > restoring that? (you aren't, and you can't if you switch out of FIQ mode.) > This is the fundamental problem with the approach of treating FIQs as if > they were IRQs. They aren't the same because the processor doesn't give > the same guarantees. Does "Basically to switch modes, we *HAVE* to clobber one register" mean that to switch processor mode we _must_ use "movs pc, lr" or "ldmia sp, {r0-pc}^" instruction only? I mean, those instructions demand SPSR_current to be clobbered (that is SPSR_current's control bits M[4:0] should be equal to the next processor mode) before executing the instruction. Is there any other problematic scenario except this: 1) USR mode ----(syscall)----> 2) SVC[1] mode ----(before any instruction is executed)----> 3) FIQ mode ----(save r0, spsr and lr through 'vector_stub fiq...')----> 4) SVC[2] mode ----(take care of FIQ stuff and get back through "movs pc, lr" to previous mode)----> 5) SVC[1] mode :-( but SPSR_svc is clobbered in the previous step due to "movs pc, lr" :-( :-( :-( I am aware of fiq-engine by Alessandro Rubini that runs FIQ exception handler in FIQ processor mode only: vector_fiq: - disable_fiq + ldr r9, 1f + ldr r9, [r9] + movs r9, r9 + beq fiq_ret + mov r8, lr + mov lr, pc + mov pc, r9 /* jump to userptr */ + mov lr, r8 +fiq_ret: subs pc, lr, #4 Just trying to understand the interesting stuff... Any feedback is *greatly* appreciated!! Thanks, --- Kosta

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Memory mapping in X86
by Sathishkumar Duraisamy Nov. 14, 2011

Nov. 14, 2011

Hi, I trying to implement mmap method (And by the way I writing driver to model adc). I had implemented as shown below. I am not getting the result as I expected. But got some error: map pfn RAM range req uncached-minus for 36a0e000-36a0f000, got write-back. In user space, I call mmap as follows: map_ptr = (signed short *) mmap(0,ADC_USER_MEM_SIZE,PROT_READ, MAP_SHARED,fd,0); Am I doing anything wrong here? static int simdrv_mmap(struct file *filp,struct vm_area_struct *vma) { unsigned long off = vma->vm_pgoff << PAGE_SHIFT; unsigned long physical = (__pa((void *)adc_dev.adcMemoryArea) >> PAGE_SHIFT) + off; // adcMemory I got via __get_free_pages(GFP_KERNEL, 1); unsigned long vsize = vma->vm_end - vma->vm_start; unsigned long psize = __pa((void *) adc_dev.adcMemoryArea) - off; // Module spanning too high if (vsize > psize) { printk(KERN_ALERT "Module spanning too high"); return -EINVAL; } vma->vm_flags |= VM_IO; vma->vm_flags |= VM_RESERVED; vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot); if (remap_pfn_range(vma,vma->vm_start, physical,vsize,vma->vm_page_prot)) { return -EAGAIN; } printk(KERN_ALERT "Sucessfully memory mapped"); return 0; } -- Regards, Sathishkumar D http://flowersopenlab.weebly.com/

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why does the kernel use mutex to protect the address_space structure
by Zheng Da Nov. 13, 2011

Nov. 13, 2011

Hello, I try to understand why the kernel uses mutex to protect the address_space structure. I checked the critical areas protected by i_mmap_mutex, and didn't see these areas use IO. I don't see why we can't use spin locks. Do I miss something? It seems most of critical areas provided by i_mmap_mutex aren't very large. Thanks, Da

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pseudo device
by Srinidhi Rao Nov. 13, 2011

Nov. 13, 2011

Well I would like to start learning some kernel level programming and try my hands at devices. I want to know what is a pseudo device (yeah.. the name suggests something vaguely :), how does the pseudo device actually get marked in the kernel and is there a way to connect o pseudo device externally also. TIA -- Best Regards, Srinidhi. ============================================== The true delight is in the finding out rather than in the knowing. -- Isaac Asimov ==============================================

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Obtaining a list of open sockets from "struct task_struct"
by Vimal Nov. 13, 2011

Nov. 13, 2011

Hi all, I am trying to understand more about kernel data structures and I would like to know how to obtain a list of TCP/UDP sockets, starting from a "struct task_struct" variable "task". So far, I have understood the following: Please correct me if I am wrong! :) - An open file descriptor is represented by a "struct file *" in the tasks's file table: task->files.fdt.fd - The file tables are organised as a linked list - The file table contains a structure fd_array, that is an array of "struct file *", each representing an open file But, almost all operations in the TCP code start from a "struct sock". Are the "struct file" and "struct sock" somehow connected? Thanks, -- Vimal

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Pin configuration
by Stefan Schick Nov. 11, 2011

Nov. 11, 2011

Hey, I am currently working on a TDM3730 in combination with a Blizzard Base board (http://denx-cs.de/?q=tdm3730) It is my goal to connect a CAN-Bus and get it up and running. As driver we use the SJA1000 OF platform which is already in my Kernel. For that we have to build a little custom board with the SJA1000 Chip and attach it to the GPIO's or in our case the GPMC's. The Pin configuration is stored in the u-boot source at "u-boot.git/board/technexion/tdm3730". I also found the code to override the Pin configuration in the architecture specific code in the Kernel. What I don't get is what happens between the Architecture specific Pin configuration and the SJA1000 Driver. How does the Driver know which Pins to use? Regards, Stefan

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About kernel memory limit
by Geraint Yang Nov. 11, 2011

Nov. 11, 2011

Hi there, I am a newbie to Linux kernel programming. I am going to make a module which will cost much memory in kernel, I just want to know how much memory I can get by calling memory allocate API in kernel. Any help will be appreaciated.

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Re SD/MMC subsystem resources
by Asutosh Das Nov. 11, 2011

Nov. 11, 2011

Hi All Please can you let me know if there are any good articles and/or books that explain the linux sd/mmc stack that you may know of. TIA -- ~/asd

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